Tertiary isononanoic acids are understood to mean positional isomers of aliphatic carboxylic acids having nine carbon atoms in the molecule, in which the α-carbon atom adjacent to the carboxyl group bears two alkyl groups in addition to the main chain. This α-carbon atom bears a total of four substituents and is often also referred to as a quaternary carbon atom. An industrially important representative of such a positional isomer of isononanoic acid is 2,2,4,4-tetramethylpentanoic acid. Preparation of the latter is carried out by hydroxycarbonylation or the so-called Koch reaction of industrially available diisobutene with sulfuric acid at elevated carbon monoxide pressure. (Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., Vol. 6, pp. 502-503, 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).
Also of industrial importance are the so-called Versatic acids 9 from Momentive Specialty Chemicals Inc., the structure of which may be described as follows:
where R1 and R2 are alkyl groups comprising 6 carbon atoms in total.
In addition to the use of the dimers of isobutene, the dimers of n-butene are also described as starting material for the preparation of tertiary isononanoic acids. According to DE 199 06 518 A1, octenes from the dimerization of n-butenes are also converted with carbon monoxide and water to the tertiary isononanoic acids by means of the Koch reaction catalyzed by strong acids. The composition of the isononanoic acid mixture depends on the isomeric composition of the di-n-butene. In addition to an isononanoic acid mixture obtained from the Koch reaction of an unfractionated di-n-butene, DE 199 06 518 A1 also describes the composition of isononanoic acids based on di-n-butenes which are obtained as a more highly branched top fraction and as a less branched bottom fraction during the fractionation of di-n-butene. The reported isononanoic acid mixtures comprise 2-ethyl-2-methylhexanoic acid as the main component in varying amounts. The published German patent application DE 10 2013 009 323 A1 likewise relates to the production of a mixture comprising tertiary isononanoic acids proceeding from an octene mixture as starting material. The octene mixture is obtained by the dehydration of 2-ethylhexanol and is subsequently reacted with carbon monoxide at elevated pressure in the presence of acidic catalysts.
The mixtures of tertiary isononanoic acids obtained are useful intermediates in industrial organic chemistry due to their high degree of branching at the α-carbon atom, which are processed to give a multitude of conversion products for a wide variety of different fields of use. For example, the salts thereof are used as drying accelerators or siccatives for coatings.
The esters of the tertiary isononanoic acids have a high resistance to hydrolysis and oxidation, and the polyol esters thereof, their glycol and polyglycol esters for example, are used as hydraulic fluids and lubricants. Tertiary isononanoic acids may also be derivatized to vinyl esters, either by reaction with acetylene in the presence of zinc salts at temperatures of 200 to 230° C. or by the transition metal-catalyzed transvinylation reaction with a vinyl ester of another carboxylic acid, usually vinyl acetate or vinyl propionate (Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., Vol. 38, pp. 70-73, 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). Vinyl esters have a certain commercial significance as the monomer component in homo- and copolymers. As comonomers, they can modify the properties of polymers such as polyvinyl acetate, polyvinyl chloride, polystyrene or polyacrylic esters. The corresponding copolymers can be processed to give paints which feature an improved resistance to hydrolysis and relatively low moisture absorption. Vinyl esters of tertiary isononanoic acids are also used for preparing adhesives. Tertiary isononanoic acids can also be converted to glycidyl esters, for example by reaction with epichlorohydrin in the presence of sodium hydroxide by the method described in U.S. Pat. No. 6,433,217 B1 (Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 5, John Wiley & Sons 1979). Industrially, glycidyl esters of tertiary carboxylic acids are used, for example, for modifying alkyd resins or for producing color-stable coating compositions for exterior applications. (Weissermel, Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], VCH Verlagsgesellschaft, 3rd edition, 1988, page 152; Ullmann's Encyclopedia of Industrial Chemistry, 6th Ed., Vol 24, Wiley-VCH-Verlag GmbH 2003, page 643).
It is also known that the reaction of olefins to give a mixture of tertiary carboxylic acids in strongly acidic medium is possible, even without applying pressure, when carried out in the presence of concentrated formic acid as carbon monoxide source. This variant, known also as the Koch-Haaf method, may also be extended successfully even to alcohols as starting substances. (Falbe, Carbon Monoxide in Organic Synthesis, Springer Verlag Berlin, Heidelberg, N.Y., 1970, pages 137-143; Angew, Chem., 70, 1958, page 311; Fette, Seifen, Anstrichmittel [Fats, Soaps, Paints] 59, 1957, pages 493-498). According to Liebigs Ann. Chem. 618, 1958, pages 251-266, the reaction of 2-methylbutanol affords, in addition to the expected 2,2-dimethylbutyric acid, other acids such as trimethylacetic acid and C7 acids and also a considerable proportion of higher, mainly C11 acids, which may be derived from a pentene dimer.
The raw material used for the industrial preparation of tertiary isononanoic acids is the C4 cut from the steamcracking of naphtha. The availability thereof compared to the C2 and C3 cracking products can be controlled by the conditions of steamcracking and is guided by the market conditions.
1,3-Butadiene is first removed from the C4 cracking products by extraction or by selective hydrogenation to n-butenes. The resulting C4 raffinate, also called raffinate I, comprises predominantly the unsaturated butenes isobutene, 1-butene and 2-butene, and the hydrogenated products n-butane and isobutane. Isobutene is removed from raffinate I in the next step, for example by reversible proton-catalyzed addition of water to give tertiary butanol, and the resulting isobutene-free C4 mixture is referred to as raffinate II. Isobutene can be recovered again from tertiary butanol by redissociation (Weissermel, Arpe, Industrielle Organische Chemie, VCH Verlagsgesellschaft, 3rd Edition, 1988, pages 74-79). It is likewise possible to contact the butadiene-free C4 raffinate at elevated temperature and under pressure with an acidic suspended ion exchanger. Isobutene oligomerizes to diisobutene, triisobutene, and in a small proportion to higher oligomers. The oligomers are separated from the unreacted C4 compounds. It is then possible to obtain diisobutene or triisobutene in pure form by distillation from the oligomerized material. The dimerization of n-butenes with isobutene forms co-dimer to a small degree (Weissermel, Arpe, Industrielle Organische Chemie, VCH Verlagsgesellschaft, 3rd Edition, 1988, p. 77; Hydrocarbon Processing, April 1973, pages 171-173).
Diisobutene, either prepared by the oligomerization of pure isobutene obtained by redissociation or obtained in the course of workup of a butadiene-free raffinate I, is then converted to a C9 derivative lengthened by one carbon atom. In this case, diisobutene is converted by the abovementioned hydroxycarbonylation or Koch reaction with carbon monoxide and water in the presence of sulfuric acid to the highly branched isononanoic acid 2,2,4,4-tetramethyl-1-pentanoic acid. Owing to the double alkyl branch at the carbon atom adjacent to the carboxyl group, this positional isomer of isononanoic acid is frequently also referred to as neononanoic acid.
The n-butenes present in raffinate II after the isobutene removal are also converted industrially to butene oligomers, from which isomeric octenes are separated, and these are converted via hydroxycarbonylation to the corresponding isononanoic acids (DE 199 08 320 A1; DE 199 06 518 A1). The oligomerization of n-butenes is operated industrially by the DIMERSOL process or by the OCTOL process.
Against the background that the availability of octenes based on the C4 cut from naphtha cracking is limited and depends on the local site conditions, it is desirable to develop further C8 sources based on inexpensively available large-scale products which can be transported to various sites in a simple manner. 2-Ethylhexanol is available inexpensively as an industrial large-scale product which can be sold widely without any problems. As is well known, 2-ethylhexanol is prepared on the industrial scale by hydroformylation or oxo process using propylene to give n-butyraldehyde with subsequent alkali-catalyzed aldol condensation to give 2-ethylhexenal followed by full hydrogenation to give 2-ethylhexanol (Ullmanns Encyklopädie der technischen Chemie [Encyclopedia of Industrial Chemistry], 4th Edition, 1974, Verlag Chemie, Volume 7, pages 214-215). The utilization of 2-ethylhexanol as the C8 source enables the provision of tertiary isononanoic acids based on propylene and reduces dependence on C8 availability based on butene.
The preparation of tertiary isononanoic acids from 2-ethylhexanol is described in DE 1 142 167. According to the known method, a mixture of 2-ethylhexanol and sulfuric acid under carbon monoxide pressure is passed over trickle elements and predominantly 2-ethyl-2-methylhexanoic acid is obtained. The known method operates at high pressures of 500 atm.